1. Field of the Invention
This invention relates to a telemetric system for transmitting seismic data from remote sensors to a central recording unit.
2. Description of the Prior Art
Recent developments in seismic exploration demand the use of several hundred to a thousand seismic sensor groups, each composed of one or more individual sensors. Usually a group will contain three to 30 sensors electrically interconnected to form a single data channel. Formerly it was necessary to provide a multiconductor seismic cable containing many hundred conductor pairs, one pair for each sensor group, to transmit the seismic-data analog signals from the sensor groups to a central multichannel data processing and recording unit. Because of the sheer bulk and weight of such a cable, geophysicists have devised various telemetric systems. Usually a typical system include one or more channels for transmission of interogation and command signals and a channel for data transmission.
A typical telemetric system is disclosed in U.S. patent application Ser. No. 664,618, now U.S. Pat. No. 4,092,629, for a Decentralized Seismic Data Acquisition System, assigned to the assignee of this invention. Briefly, the above-cited system is designed primarily for marine seismic exploration. It consists of a plurality of cable sections, such as 50, each section being about 200 feet long. The cable sections are mechanically and electrically coupled together end-to-end by connector modules to form a cable assembly.
The cable assembly is coupled to a central station mounted in a ship. The ship tows the cable assembly through the water along an assigned line of survey. The central station includes recording circuitry and apparatus to transmit control and interrogation signals through a telemetric interrogation link and to receive, process, and record digital words from the telemetric data link.
Embedded in each cable section are 10 to 12 elemental seismic sensor groups each of which is composed of several interconnected seismic sensors such as hydrophones. The electrical output of each sensor group constitutes a single data input channel. Contained within each connector module, is a 12-channel transceiver unit. The seismic sensor groups in a given cable section are connected to the respective inputs of the transceiver unit associated with that section.
The transceiver unit includes a multiplexer coupled to the interrogation link, for receiving the analog signals from the respective sensor groups. In response to interrogation signals transmitted from the central recording station, the multiplexer is reset and then advances to a first input channel to acquire a first analog input signal sample from a seismic sensor group. The signal sample is filtered, gain conditioned, digitized as a digital data word, and clocked out over the data link to the central recording station. As the interrogation signal arrives successively at the various transceiver units, a corresponding digital data word is transmitted back to the recording station.
A second interrogation pulse sequences the multiplexers in the fifty data transceivers to a second input channel for sampling and digitizing to provide a second digital data word from the respective second channels. Over a period of one scan cycle, which is approximately 1 millisecond (thousandth of a second), all of the input channels from all of the data transceivers are sampled.
During each scan cycle, approximately 500 to 600 multibit data words are sent to the central recording station by employing a two-level time-sequential/channel-sequential multiplexer scheme. The digital data words transmitted from the respective transceiver associated with each cable section are ordered in accordance with the propagation delay time for a signal travelling through the interrogation link between the central-station recorder and the transceivers. Digital data words from corresponding channels within the transceiver units are ordered in accordance with the channel-select sequence during a scan cycle.
The physical cable assembly just described is specifically designed for use in the water. Once the cable sections have been joined together, they are not readily disconnected. Accordingly, the entire cable assembly, nearly two miles long, is stored on a large reel on the stern of a ship. In use, the cable is deployed in the water and towed behind the seismic recording ship. Specially-built for marine use, each section weighs about 500 pounds in air but is neutrally bouyant in the water. The central recording unit, mounted in the ship is always connected to the leading end of the cable assembly. The cable sections are polarized; that is, the cable sections are always mated together in only one way with respect to each other. There is a leading end and a trailing end to each cable section and connector module.
One object of the present invention is to design a cable having operating principles similar to the marine cable for use on land. The electrical aspectss of data processing and data telemetry between sensors and the recording unit apply equally to land or marine use. But the physical construction of the cable sections and certain electronic circuits in the connector modules must be modified for use on land.
In land operations, the cable sections are laid along the ground, end-to-end. Each end of a section is terminated by a male plug. Transceiver modules having two female plugs, mateable with the male cable plugs, electrically interconnect adjacent cable sections. The entire array of interconnected cable sections is termed a "spread". As the exploration effort progresses along an assigned line of survey the spread must be advanced along the ground. After recording operations are completed at a given station, the spread is advanced by disconnecting one or more sections and transceiver modules from the back end of the spread and reconnecting those sections and modules to the front end of the spread in the direction of advance. Thus, the cable sections and transceivers modules must be readily disconnectable.
In many areas, particularly in harsh terrain, the sections are moved by hand by unskilled laborers. Accordingly, the cable sections and transceiver modules must be light enough to be hand-carried. Because of the use of unskilled laborers who cannot distinguish one end of a cable from the other, the cables should advantageously be unpolarized. That is, the cable sections and transceiver modules can face in either direction relative to one another and to the recording equipment, which is usually mounted in a wheeled vehicle although the recording equipment also, may be man-portable. It is commonplace to lay out a very large number of cable sections ahead of time and only utilize a fraction of these sections as the actual speed to be associated with a particular survey station during a particular recording cycle. The recording unit is plugged into the spread at any convenient location. There is no a priori knowledge as to which end of the spread is the leading or trailing end with respect to the recording unit and the direction of advance.
In the marine system, separate telemetric links were used to send command, interrogation and data signals. In the interest of weight saving, only one telemetric link is used in the land system for all three signals. Accordingly, since the cable sections and transceiver modules are unpolarized with respect to the physical location of the recording unit, a given transceiver module must be told which way to "face" to enable it to receive interrogation and command signals from the recording unit and to transmit data signals back to the recording unit.
Representative land telemetric systems are described in U.S. Pat. No. 3,652,979 for "Installation for the Transmission of Multiplexed Seismic Signals"; U.S. Pat. No. 3,873,961, for "Method and Apparatus for Synchronizing Modular Seismic System"; U.S. Pat. No. 3,911,226, for "Installation for Multiplex Transmission of Digital Signals"; U.S. Pat. No. 3,881,166, for "Data Array Network Systems".